1. Field of the Invention
The invention relates to thin films of magnetically soft alloys. More particularly, the invention relates to articles comprising these alloys and methods for making such articles.
2. Description of the Related Art
Thin film soft magnetic materials are useful in modern, high-frequency, electromagnetic devices, e.g., as a field-amplifying component in the read-write head of magnetic disk memories in computers or as a core in microtransformers and inductors. Among the desired properties of these films are relatively high saturation magnetization (4.pi.M.sub.s), low coercivity (H.sub.c), high permeability, high electrical resistivity and high corrosion resistance. Various applications of soft magnetic thin films are described, e.g., in books Magnetic Thin Films by R. F. Soohoo, Harper and Row, 1965; Thin Ferromagnetic Films by M. Prutton, Butterworth, 1964; and in articles such as C. R. Sullivan and S. R. Sanders, IEEE Trans. on Power Electronics, Proc. 24th Annual Power Electronics Specialists Conf., p. 33-40, June 1993; and T. Yachi et al., IEEE Trans. Magn. 28, 1969-1973 (1992).
Among conventional soft magnetic thin films, nickel-iron (Ni--Fe) based films such as 80% Ni-20% Fe (permalloy) are useful because of their favorable magnetic properties and zero magnetostriction characteristics. Iron-based films such as iron-tantalum (Fe--Ta), iron-zirconium (Fe--Zr) and iron-hafnium (Fe--Hf) alloys generally exhibit saturation magnetizations (4.pi.M.sub.s) of approximately 15-20 kilogauss (kG) as compared to approximately 10 kG for the 80% Ni permalloy films (see, e.g., N. Kataoka et al., Japanese J. Appl. Phys. 28, L462-L464, 1989, Trans. Jap. Inst. Metals 31, 429, 1990). However, iron-based films exhibit poorer soft magnetic properties and require post-deposition heat treatment.
To obtain improved soft magnetic properties, nitrogen-containing films of these iron-based alloys such as iron-tantalum-nitrogen (Fe--Ta--N) have been prepared. See, e.g., E. Haftek et al., IEEE Trans. Magn. 30, 3915-3917 (1994); N. Ishiwata et al., J. Appl. Phys. 69, 5616 (1991); J. Lin et al., IEEE Trans. Magn. 30, 3912-3914 (1994); and G. Qiu et al., J. Appl. Phys. 73, 6573 (1993). However, although desirable magnetic softness, e.g., a coercivity (H.sub.c) of less than approximately 2 oersteds (Oe) (for microtransformer applications), is obtainable in these nitrogen-containing films, it is apparent from the aforementioned articles that such desirable soft magnetic properties are difficult to obtain in an as-deposited form, but are possible after post-deposition heat treatment at high temperatures.
However, such heat treatment of deposited films is an additional processing step that needs to be avoided if possible, not only from a manufacturing cost point of view but also because of the complications of having to expose various other components and materials in the devices to high temperatures. Therefore, it is desirable for the required soft magnetic properties in the films to be obtained in the as-deposited condition, or at worst, with a very low temperature heat treatment below approximately 150.degree. Celsius.
Desirable high-frequency properties for soft magnetic films include relatively high permeability and low power loss. There are several sources of loss in ferromagnetic materials, including hysteresis loss, eddy current loss and ferromagnetic resonance (FMR) loss.
Hysteresis loss is reduced or minimized, e.g., by avoiding magnetic domain wall displacement, such as by performing alternating current (AC) operation of the magnetic films in the magnetically hard-axis so that magnetization only by spin rotation is used. Such mode of operation is accomplished most conveniently by providing a strong anisotropy (H.sub.k) and a square magnetic hysteresis (M-H) loop. For example, see co-pending application Ser. No. 08/595,543, filed Feb. 2, 1996 now U.S. Pat. No. 5,780,175 and assigned to the assignee of the present invention.
Eddy current loss increases in proportion to the square of the operating frequency, and thus plays an important role in the high-frequency applications. Eddy current loss is reduced, e.g., by increasing the field penetration depth (skin depth) with relatively high electrical resistance in the magnetic material, either by using a thin film configuration or by selecting relatively high resistivity materials.
The occurrence of ferromagnetic resonance (FMR) in high-frequency ranges such as approximately 10 megahertz (MHz) or greater in most of the soft magnetic materials generally causes the magnetic permeability to drop off and the magnetic loss to increase by orders of magnitude, often spanning a frequency range from approximately 1-2 orders of magnitude. Such behavior is conventional for Ni--Zn ferrites. For example, see generally R. S. Tebble and D. J. Craik, Magnetic Materials (Wiley, New York, 1969), p. 598.
Ferromagnetic resonance (FMR) occurs when the frequency of the applied AC field matches the characteristic precession frequency of spins in the magnetic material. This precession frequency, fr, is determined by the total anisotropy field (H.sub.k) experienced by the spins. For a thin film, for which the demagnetizing field (H.sub.d) along the z direction is approximately equal to the saturation magnetization (4.pi.M.sub.s), the FMR frequency is expressed as EQU f.sub.r =2.pi..gamma..multidot.(H.sub.k .multidot.4.pi.M.sub.s).sup.1/2,
where .gamma. is the gyromagnetic constant (2.pi..gamma.=2.8 MHz/Oe), and H.sub.k is the in-plane anisotropy field. This relation applies for the case when the AC field is applied in the plane of the film perpendicular to the easy-axis. In order to raise the FMR frequency so that ferromagnetic resonance does not interfere with the high-frequency operation of the magnetic materials, a higher anisotropy field (H.sub.k) and higher saturation magnetization (4.pi.M.sub.s) are needed. Typically, values of the anisotropy field (H.sub.k) up to approximately 10-15 oersteds (Oe) are obtainable, but anisotropy fields (H.sub.k) higher than 15 Oe in soft magnetic films generally are difficult to obtain. However, in high-frequency applications such as in wireless cellular communications, magnetic films with FMR frequencies of at least approximately 1 gigahertz (GHz) are desired.
Therefore, it is desirable to have soft magnetic thin films with greater saturation magnetization (4.pi.M.sub.s) and greater anisotropy fields (H.sub.k) than conventional soft magnetic thin films.